First, make the labneh. Mix the yogurt with a pinch of salt. Line a sieve with muslin or a clean J-cloth and rest over a deep bowl. Spoon in the yogurt, cover, and put in the fridge to strain for about 4 hrs. You'll be left with a firmer, soft cheese-like substance – this is your labneh.
Heat oven to 180C/160C fan/gas 4. To spatchcock the chicken, put it on a chopping board, breast-side down, with its legs pointing towards you. Use sharp scissors to carefully cut out the backbone and parson’s nose in one piece (you can freeze them to use in stock). Turn the chicken over and press on the breastbone to flatten it – you should feel it breaking.
Mash the butter with the tarragon and wild garlic and season. Use your fingers to ease the skin from the breast of the chicken and spread the butter over the breast under the skin. Season the bird and rub with any remaining butter and a little olive oil. Dust with the za’atar.
In a roasting tin, combine the lemon slices, garlic, red onions,chickpeas and bay leaves with 1 tbsp olive oil. Season and sit the bird on top. Roast for 45-50 mins, basting with the juices a couple of times, until a thermometer inserted into the thickest part of the breast reads 75C, or the juices run clear and theflesh is opaque. Remove from the oven and rest for at least 15 mins.
Turn the labneh out into a bowl. In a mini chopper or small blender, blitz the wild garlic with the olive oil and lemon juice, then pour this on top of the labneh. Carve the chicken once rested and divide between plates with the wild garlic labneh and chickpeas.